FMI
Image Data Processing
IODP drilling and logging
operator: CDEX
Hole: C0020A
Expedition: 337
Location: Offshore Shimokita Peninsula (Japan Sea)
Latitude: 41° 10.5983' N
Longitude: 142° 12.0328' E
Logging date:
Sea floor
depth (driller's):
1208.5 m DRF
Sea floor
depth (logger's):
1208.5 m WRF
Total
penetration: 3674.5 m DRF (2466 m DSF)
Total core
recovered: 198.4 m (75.3 % of the 263.5 m cored interval)
Oldest sediment recovered: Oligocene-Early Miocene
Lithologies: Diatom-rich silty clay, shale, sandstone, siltstone, coal-rich horizons
FMI Main Pass:
FMI Repeat Pass:
Magnetic declination: -8.59569°
The
FMI (Formation MicroImager) maps the conductivity of the borehole wall with a
dense array of sensors. This provides a high resolution electrical image of the
formation which can be displayed in either gray or color scale. The purpose of
this report is to describe the images from IODP Expedition 337 and the steps
used to generate them from the raw FMI measurements.
The FMI provides an electrical borehole image generated from 24 button electrodes (mounted on each of the four pads and flaps) and 192 microresistivity measurements. The tool provides approximately 80% coverage of the borehole wall in a 8-inch diameter borehole. The tool string also contains a triaxial accelerometer and three flux-gate magnetometers (in the GPIT, General Purpose Inclinometry Tool) whose results are used to accurately orient and position the images. Measurements of hole size, cable speed, and natural gamma ray intensity also contribute to the processing.
Data
Quality
The FMI images are generally of high quality for both main and repeat passes; locally, however, noise has been observed on the images taken from the electrode flaps of the FMI due to weaker contact with the borehole wall. In general, the pads make a better contact with the borehole wall, resulting in higher quality images.
Image Processing
Processing is required to convert the electrical current in the formation, emitted by the FMI button electrodes, into a gray or color-scale image representative of the conductivity changes. This is achieved through two main processing phases: data restoration and image display.
1) Data Restoration
Speed correction
The data from the z-axis accelerometer is used to correct the vertical position of the data for variations in the speed of the tool ('GPIT speed correction'), including 'stick and slip'. In addition, 'image-based speed correction' is also applied to the data: the principle behind this is that if the GPIT speed correction is successful, the readings from the two rows of buttons on the pads will line up, and if not, they will be offset from each other (a zigzag effect on the image).
Equalization
Equalization is the process whereby the average response of all the buttons of the tool are rendered approximately the same over large intervals, to correct for various tool and borehole effects which affect individual buttons differently. These effects include differences in the gain and offset of the pre-amplification circuits associated with each button, and differences in contact with the borehole wall between buttons on a pad, and between pads.
Button correction
If the measurements from a button are unreasonably different from its neighbors (e.g. 'dead buttons') over a particular interval, they are declared faulty, and the defective trace is replaced by traces from adjacent good buttons.
EMEX voltage correction
The button response (current) is controlled by the EMEX voltage, which is applied between the button electrode and the return electrode. The EMEX voltage is regulated to keep the current response within the operating range. The button current response is divided by the EMEX voltage to give the relative conductivity of the formation.
Depth-shifting
Each of the logging runs is depth-shifted to the sea floor (-1208.5 m WRF).
A high-resolution conductivity log is then produced from the FMI data by averaging the conductivity values from the 192 button electrodes. This enables the FMI data to be plotted using common graphing applications and more easily used in numerical analyses (e.g. spectral analysis). Specifically, the FMI conductivity values are averaged over each of the four pads and flaps and over five 0.0254-cm depth levels to produce a file with 1.27-cm sample interval containing the total (4-pad and flpas, 192-button) average conductivity value, plus the 48-button averages from each of the four pads and flaps. Note that the conductivity values are un-scaled and more accurate (but lower vertical resolution) values are given by the resistivity logs from the HRLA resistivity tool.
2) Image Display
Normalization
Once the data is processed, both 'static' and 'dynamic' images are generated; the differences between these two types of image are explained below. Both types are provided online.
In "static normalization", a histogram equalization technique is used to obtain the maximum quality image. In this technique, the resistivity range of the entire interval of good data is computed and partitioned into 256 color levels. This type of normalization is best suited for large-scale resistivity variations. The image can be enhanced when it is desirable to highlight features in sections of the well where resistivity events are relatively subdued when compared with the overall resistivity range in the section. This enhancement is called "dynamic normalization". By rescaling the color intensity over a smaller interval, the contrast between adjacent resistivity levels is enhanced. It is important to note that with dynamic normalization, resistivities in two distant sections of the hole cannot be directly compared with each other. The images are normalized with a 2-m interval for dynamic normalization.
Oriented presentation
The normalized images are then converted to gif files using in-house code. Thery are posted online and displayed as an unwrapped borehole cylinder. Several passes can be oriented and merged together on the same presentation to give additional borehole coverage if the tool pads and flaps followed a different track. A dipping plane in the borehole will be displayed as a sinusoid on the image; the amplitude of this sinusoid is proportional to the dip of the plane. The images are oriented with respect to north, hence the strike of dipping features can also be determined.
Additional information about the drilling and logging operations can be found in the Operations and Downhole Measurements sections of the expedition report, Proceedings of the Integrated Drilling Program, Expedition 337.
For questions about the database, please contact:
Cristina Broglia
Phone: 845-365-8343
Fax: 845-365-8777
E-mail: Cristina Broglia
For questions about the logs, please contact:
Yoshinori Sanada
E-mail: sanada@jamstec.go.jp
Yukari Kido
Email: ykido@jamstec.go.jp
Yuichi Shinmoto
Email: shinmoto@jamstec.go.jp